In principle, a stored-program-controlled telephone exchange includes four part-systems, namely a first part-system comprising a plurality of line circuits to which incoming subscriber lines are connected and which further include program logic for monitoring and controlling the connected line circuits. Each subscriber who subscribes to the local exchange is assigned a line circuit. The second part-system includes a plurality of exchange terminals to which external transmission lines to other telephone exchanges are connected. The channels on the transmission lines are grouped in routes with a common point of origin and a common destination. The third part-system includes a switching device, normally a group selector, partly for connecting physically together two lines included in the first part-system, an internal connection, and partly for physically connecting a line in the second part system with a line in the first part-system, a terminating connection, and partly for connecting physically one line from the first part-system with a line from the second part-system, an originating connection. Thus, an internal connection means that a path is established between two internal subscribers of the local exchange. An originating connection means that an internal subscriber of the local exchange is connected to a subscriber who belongs to a different telephone exchange, whereas a terminating connection means that an external subscriber is connected to an exchange internal subscriber.
The fourth part-system of the telephone exchange includes solely program logic which functions to coordinate computer control of the three part-systems. Data records are found in different memory areas and store addressers or identifiers by means of which the program logic is able to identify and control trunk circuits and switching devices. Data records containing address information for, e.g., incoming exchange terminals are linked to data records for outgoing exchange terminals with the aid of separate addressers or identifiers, called pointers. These pointers are used by the program logic in a known manner to identify exchange terminals and switching elements which need to coact when establishing a connection path between two line circuits or between a line circuit and an incoming or an outgoing exchange terminal.
Hardware is linked to data records in the same manner as different data records in different memory areas are linked with pointers. The program logic is thus able to identify and therewith control the hardware linked to a particular data record.
In the case of an originating connection, a data record stored in the first part-system and corresponding to a line circuit is linked to a data record stored in the second part-system and corresponding to an outgoing exchange terminal via a coordinated data record of a first type in the fourth part-system. In the case of a terminating connection, on the other hand, a data post stored in the second system and corresponding to an incoming exchange terminal is linked to a data record stored in the first part-system and corresponding to an outgoing line circuit, this linking also being achieved via a coordinating data record of the first type in the fourth part system. These coordinating data records are normally referred to as call handlers. These call handlers are seized dynamically when dialing, or key sending, a telephone number and contain a large amount of information, among other things information relating to the telephone numbers of the parties concerned with the call, the category, route, multiple group selector states, etc.
If a subscriber wishes to be connected to a conference telephone system, i.e. a system which enables several parties to converse with one another simultaneously, new connection cases arise, e.g. in the three-party case, namely
standard three-party with all three exchange subscribers internal; PA1 standard three-party with two of the subscribers being internal and a third external, the connection path thus including an exchange terminal; PA1 standard three-party with one internal subscriber and two external subscribers, the connection path therewith including two exchange terminals.